What recipe did you follow? If you used extract, you can pin-point the gravity based on how much water you used and the amount of extract. Piece of cake. If you did a partial boil then measured the gravity, there is a good (great) chance that it wasn't mixed sufficiently.
If you did AG, then your efficiency could be off.
Either way, even though you didn't specify what gravity you measured (and was low) , I wouldn't recommend adding anything at this point. If you added sugar, the yeast will eat it, up to the point that they stall from lack of nutrients, oxygen, or just high ethanol content. IMHO, it's better to have a little less than what you thought than just add sugar and make it a hoochy foul tasting "strong" beer.